1. Field of the Invention
This invention resides in the field of test devices for clinical use that provide visual readouts, and is particularly concerned with test devices that utilize a calorimetric indicator to display an icon or symbol indicating either the presence or absence of a particular substance in a sample, a high or low concentration of a substance, a high or low pH, or any such indication of either a desired or undesired condition of the sample.
2. Description of the Prior Art
Test devices that produce visually detectable plus signs to indicate the presence of bacterial vaginosis in a sample of vaginal fluid are disclosed in the following United States patents (inventors: Paul J. Lawrence et al.): U.S. Pat. No. 5,660,790, issued Aug. 26, 1997; U.S. Pat. No. 5,897,834, issued Apr. 27, 1999; U.S. Pat. No. 5,910,447, issued Jun. 8, 1999; U.S. Pat. No. 6,099,801, issued Aug. 8, 2000; U.S. Pat. No. 6,113,856, issued Sep. 5, 2000; and U.S. Pat. No. 6,200,817, issued Mar. 13, 2001. Two distinct tests are disclosed in these patents: a pH test of vaginal fluid in which a pH of 4.7 or greater serves as one indication of an infection, and an amine test in which the presence of volatilizable amines serves as another indication of an infection. The devices in these patents are laminated test cards and the tests are conducted by applying a specimen of vaginal fluid to the surface of a card with a swab and checking the card for the appearance of symbols on the card surface. In each of these two tests, a negative test result appears as a “minus” sign formed by a single horizontal bar and a positive result appears as a “plus” sign formed by two bars crossing at right angles. The plus sign is formed by two separately operable but intersecting bars: one horizontal (the positive control); and, the second vertical (the test response). The horizontal bar thus appears regardless of the outcome of the test, and thereby serves as a positive control indicating that the indicator that causes the color change in both bars is functioning properly. Thus, if no bar at all appears after application of a sample, the test device is deemed not usable and should be discarded; the appearance of a single bar (a “minus” sign) means that the device is functional and the test result is negative; and the appearance of both bars (to form the “plus” sign) indicates that the device is functional and the test result is positive.
Plus and minus signs are also produced by the test devices disclosed in U.S. Pat. No. 4,916,056, issued Apr. 10, 1990, and U.S. Pat. No. 5,008,080, issued Apr. 16, 1991, both to inventors William E. Brown, III, et al. The tests in these patents involve immunological binding rather than pH changes, but like the Lawrence et al. patents, the tests results are displayed as a plus sign formed when two separately operable but intersecting bars become visible and a minus sign formed from when only one of the two bars becomes visible. All patents in this and the preceding paragraph are incorporated herein by reference in their entirety.
Licensed physicians and laboratory technicians find products like these useful and efficient because of their compact nature and their simple visual readout. Since they are easy to use, the devices are also attractive for consumers and other individuals who are not clinically trained. This is particularly true of the Lawrence et al. devices where the test procedures consist of simply applying a swab of the sample to the surface of the device, and watching the device to see what symbols appear. In some cases, however, users, particularly untrained individuals, may read the devices incorrectly, reporting results that are either false negatives or false positives.
One source of an incorrect reading arises from the independent responses of the two independent, separable but intersecting bars and the fact that the response of each bar can vary to some degree with the amount of sample applied to the device or the amount or concentration of analyte in the sample. Slight differences in color intensity between the two intersecting bars can prompt the user to treat one bar as a color comparator for the other, and thereby wait for the color of the vertical bar to precisely match that of the horizontal bar before declaring a positive result. A positive result can thus be interpreted as negative (i.e., a false negative).
Another source of an incorrect reading occurs when the outlines of the intersecting horizontal and vertical bars are visible, even when only faintly so, before any color change has occurred. This problem can occur with any iconic readout, the terms “iconic” and “icon” referring herein to any graphical symbol whose form suggests its meaning. In the case of the Lawrence et al. and Brown et al. devices, the icon is either a minus sign indicating a negative result when only one bar appears or a plus sign indicating a positive result when both bars appear. The icon can also be a unitary plus sign that appears only as a whole and lacks the capability of allowing only the minus sign portion to appear. Icons of other forms or shapes will be readily apparent. With any of these icons, incorrect readings can arise from the construction of the device, which typically involves several laminae one or more of which forms the outlines of the icon. In some cases, for example, the icon is formed by a channeling lamina with an icon-shaped opening, overlying an indicator lamina that extends over the full lateral dimensions of the device, or at least beyond the boundaries of the opening. The channeling lamina permits the sample (and any analyte present) to contact only the portion of the indicator that is directly below the opening. Even if the channeling lamina is completely transparent, the edges of the opening can be visible to one who looks closely at the device, since the light reflectivity of the channeling lamina may differ from that of the indicator lamina. In other cases, the icon is formed by the indicator lamina itself which is in the shape of the icon and rests above any underlying layers. Although the filed behind the icon is the same color as the icon (prior to the test), the edges of the icon are faintly visible due to the difference in height, even though that difference may be very small. To the unskilled or untrained user, this visibility can suggest a positive test result when the result is actually negative (i.e., a false positive result). The thickness difference can be minimized or eliminated by depositing a material in the areas adjacent to the icon, using a material that does not change color upon contact with either the sample or the analyte. The icon and the surrounding material will then be of different chemical compositions, however. This by itself can produce enough of a color difference, however slight, to make the borders between the two visually distinguishable. Furthermore, if there is a risk of the indicator diffusing into the surrounding material and thereby obscuring the readout, precautions or structural features to prevent this from happening can also make the outline visible.